1. English-- e.g., in England, or in general.2. Scottish English: English as spoken in Scotland, with a Scottish accent.3. Scots: a Germanic language, sister to English (developed in parallel over the last 1000 years or so).4. Scottish Gaelic: a Celtic language (related to Irish Gaelic, Welsh, etc.).

Scottish Gaelic is the easiest to separate out: it's unrelated, and obviously completely distinct, and there are few speakers left (probably none are monolingual today). But of course some words have been borrowed into Scots, and then into Scottish English, and rarely even into standard English elsewhere: https://en.wikipedia.org/wiki/List_of_English_words_of_Scottish_Gaelic_origin -- apparently that even includes the word "clan"!

Scots and Scottish English are trickier to define or distinguish today, and it is often not really possible to separate them entirely, because they have effectively merged into each other. Scottish English is a dialect of English, but Scots is a different but related (sister) language; the two began to mix sometime around 1500 when English was used more in Scotland for political reasons, and they've been in intense contact every since. Scottish English pronunciation is based partly on Scots, and that is also the source for the distinct vocabulary: for example "loch" for example referring to certain lakes or seawater inlets in Scotland. And at the same time, it isn't really possible to completely distinguish between "Scottish English" and "Standard English" in Scotland. All of these varieties exist on a continuum.

The distinct spelling you mentioned for "Scottish" is probably the somewhat standardized phonetic spelling for Scots, although it can also be used to represent the dialectal pronunciation of Scottish English (or a mix of the two).